1. Field of the Invention
The present invention relates to an image heating apparatus such as a heat fixing device equipped on an image forming apparatus such as a copying machine, a printer or the like, or an apparatus for improving a surface characteristic of an image, or the like.
2. Related Art
As a heat fixing device equipped on an image forming apparatus, a heat roller system which contains therein a halogen heater, a film heating system which fixes an image by passing via a heat-resistant film a sheet in a nip formed by a ceramic heater and a pressurizing roller, an induction heating system which generates heat by causing a rotator having a metallic layer to generate an eddy current, and the like have been put to practical use or devised.
In any system, direct or indirect targets of control (i.e., a heat roller in the heat roller system, the ceramic heater in the film heating system, and a metallic roller or a metallic film in the induction heating system are controlled to maintain target temperature. If controlling the target of control to maintain the target temperature, an actual temperature of this target rises and falls around the target temperature (i.e., a temperature ripple appears). It is desirable that the temperature ripple is small to secure excellent fixation of the image. Thus, PID (Proportional, Integral and Derivative) control including PI control and PD control as disclosed in U.S. Pat. No. 5,747,774 is generally used as a control method of decreasing the temperature ripple. In the PID control, on the basis of the trend of increase and decrease of a deviation between a detected temperature and a target temperature, the control is performed by not only making an operating amount of a power control means proportional to the deviation but also adding in the factor proportional to integration of the deviation and the factor proportional to differential of the deviation. Temperature information from a temperature detecting element is sampled at a certain period (sampling period) and included in the control rule.
Incidentally, to improve gloss (a gloss level) of the fixed image and improve transparentness of the fixed image on an OHP (Overhead Projector) film, the fixing is performed generally by decelerating a fixing speed more than a usual time. Also, when a recording material such as a thick sheet so that a large amount of heat is necessary for the fixing the fixing is performed by decelerating a fixing speed more than the usual time. Thus, it is necessary to change the fixing speed according to the target of a fixing process.
However, when the temperature is controlled by the PID control, there is a drawback that the control becomes unstable if the fixing speed changes.
That is, for example, on the metallic film used in the induction heating system, the eddy current is generated in an area where magnetic flux generated by a coil acts, whereby the temperature rises. However, if the fixing speed changes, the time required so that the metallic film passes the magnetic flux action area changes according to such a change, whereby also a calorific value (or heat value) of the film changes. For example, if the fixing speed is set to be xc2xd, the magnetic flux supplied to a certain part on the film by the magnetic flux action area doubles. Therefore, if the fixing speed slows down, a temperature rise speed increases even if a power turning-on amount is the same.
That is, in case of a print mode in which the fixing speed slows down and the temperature rise speed thus increases, a temperature change of the target of control for the same operating amount is large as compared with a case where the fixing speed is fast.
Although the above PID control is effective at the specific fixing speed to suppress the temperature ripple of the target of control, a time factor is necessary, from the nature of control, for both the factor proportional to the integration of the deviation and the factor proportional to the differential of the deviation, whereby it takes a time until the temperature of the target of control settles into the target temperature because it is impossible to correspond sufficiently to a feedback speed required in the print mode in which the fixing speed is slow.
As a result, a problem that uniform gloss of the image on the surface of the recording material and/or uniform transparentness of the image on the OHP film can not be obtained is caused by the ripple (or vibration) of film temperature. Further, if the film temperature comes off from a fixable temperature area including the target temperature, a problem that defective fixing such as hot offset or cold offset arises is caused.
The present invention has been made in consideration of the above problems, and an object thereof is to provide an image heating apparatus which can secure stable fixation irrespective of a moving speed of a moving member of a heating means.
Another object of the present invention is to provide an image heating apparatus which can prevent toner offset and irregular optical transparentness of an OHP film irrespective of a fixing speed.
Still another object of the present invention is to provide an image heating apparatus comprising:
a heating means for heating an image formed on a recording material, the heating means including a moving member moving as coming into contact with the recording material;
a temperature detecting element for detecting a temperature of the heating means; and
a power supply control means for controlling an electrical power to the heating means so that the temperature detected by the temperature detecting element is maintained at a set temperature,
wherein the power supply control means sets the electrical power in accordance with the temperature detected by the temperature detecting element and a moving speed of the moving member.
Still another object of the present invention is to provide an image heating apparatus comprising:
a heating means for heating an image formed on a recording material, the heating means including a moving member moving as coming into contact with the recording material;
a temperature detecting element for detecting a temperature of the heating means; and
a power supply control means for controlling an electrical power to the heating means so that the temperature detected by the temperature detecting element is maintained at a set temperature,
wherein the power supply control means sets a sampling period of the output of the temperature detecting element in accordance with a moving speed of the moving member.
Still another object of the present invention is to provide an image heating apparatus comprising:
a heating means for heating an image formed on a recording material, the heating means including a moving member moving as coming into contact with the recording material;
a temperature detecting element for detecting a temperature of the heating means; and
a power supply control means for controlling an electrical power to the heating means so that the temperature detected by the temperature detecting element is maintained at a set temperature,
wherein the moving member can move at a first speed and a second speed lower than the first speed, and
wherein if the electrical power to be supplied to the heating means is given as A when the moving speed of the moving member is the first speed, the set temperature is given as T1, and the detected temperature of the temperature detecting element is given as T, and if the electrical power to be supplied to the heating means is given as B when the moving speed of the moving member is the second speed, the set temperature is given as T1, and the detected temperature of the temperature detecting element is given as T, B less than A is satisfied.
Further objects of the present invention will be apparent by reading the following description taken in conjunction with the accompanying drawings.